Electron beam apparatuses, in particular a scanning electron microscope (also referred to as SEM below) and/or a transmission electron microscope (also referred to as TEM below), are used to examine objects (also referred to as samples) in order to obtain knowledge in respect of the properties and behavior of the objects under certain conditions. In an SEM, an electron beam (also referred to as primary electron beam below) is generated by means of a beam generator and focused on an object to be examined by way of a beam guiding system. An objective lens is used for focusing purposes. The primary electron beam is guided over a surface of the object to be examined by way of a deflection device. This is also referred to as scanning. The area scanned by the primary electron beam is also referred to as scanning region. Here, the electrons of the primary electron beam interact with the object to be examined. Interaction particles and/or interaction radiation result as a consequence of the interaction. By way of example, the interaction particles are electrons. In particular, electrons are emitted by the object—the so-called secondary electrons—and electrons of the primary electron beam are scattered back—the so-called backscattered electrons. The interaction particles form the so-called secondary particle beam and are detected by at least one particle detector. The particle detector generates detection signals which are used to generate an image of the object. An imaging of the object to be examined is thus obtained. By way of example, the interaction radiation is x-ray radiation or cathodoluminescence. At least one radiation detector is used to detect the interaction radiation.
In the case of a TEM, a primary electron beam is likewise generated by means of a beam generator and directed onto an object to be examined by means of a beam guiding system. The primary electron beam passes through the object to be examined. When the primary electron beam passes through the object to be examined, the electrons of the primary electron beam interact with the material of the object to be examined. The electrons passing through the object to be examined are imaged onto a luminescent screen or onto a detector—for example in the form of a camera—by a system comprising an objective. By way of example, the aforementioned system additionally also comprises a projection lens. Here, imaging may also take place in the scanning mode of a TEM. As a rule, such a TEM is referred to as STEM. Additionally, provision may be made for detecting electrons scattered back at the object to be examined and/or secondary electrons emitted by the object to be examined by means of at least one further detector in order to image the object to be examined.
Combining the function of an STEM and an SEM in a single particle beam apparatus is known. It is therefore possible to carry out examinations of objects with an SEM function and/or with an STEM function using this particle beam apparatus.
Moreover, a particle beam apparatus in the form of an ion beam column is known. Ions used for processing an object are generated by means of an ion beam generator arranged in the ion beam column. By way of example, material of the object is ablated or material is applied onto the object during the processing. The ions are additionally or alternatively used for imaging.
Furthermore, the prior art has disclosed the practice of analyzing and/or processing an object in a particle beam apparatus using, on the one hand, electrons and, on the other hand, ions. By way of example, an electron beam column having the function of an SEM is arranged at the particle beam apparatus. Additionally, an ion beam column, which was already explained above, is arranged at the particle beam apparatus. The electron beam column with the SEM function serves, in particular, for examining further the processed or unprocessed object, but also for processing the object.
An object may be imaged with a high spatial resolution using an electron beam apparatus. In particular, this is achieved by a very small diameter of the primary electron beam in the plane of the object. Further, the spatial resolution may improve the higher the electrons of the primary electron beam are initially accelerated in the electron beam apparatus and decelerated to a desired energy (referred to as landing energy) at the end of the objective lens or in the region of the objective lens and the object. By way of example, the electrons of the primary electron beam are accelerated using an acceleration voltage of 2 kV to 30 kV and guided through an electron column of the electron beam apparatus. The electrons of the primary electron beam are only decelerated to the desired landing energy, with which they are incident on the object, in the region between the objective lens and the object. By way of example, the landing energy of the electrons in the primary electron beam lies in the range between 10 eV and 30 keV.
There are objects which, on account of their structure, may only be expediently examined in an electron beam apparatus if the electrons in the primary electron beam incident on these objects only have a low landing energy, for example an energy of less than 100 eV. Electrons with such low energy for example ensure that these specific objects are not destroyed and/or do not charge upon irradiation by electrons. Further, electrons at such low energies are particularly suitable for obtaining an image with a high surface sensitivity (i.e. a particularly good information content in respect of the topography and/or the material of the surface of the object) of an object to be examined.
When generating an image of the object, the user of an electron beam apparatus is always prudent to obtain the ideal image quality of an image of the object which is required for examining an object. Expressed differently, a user always wishes to create an image of the object with such a high image quality that they are able to analyze the object to be examined well on account of the image and the image information contained therein. Here, the image quality may be determined by means of e.g. objective criteria. By way of example, the image quality of an image becomes better with increasing resolution in the image or with increasing contrast. Alternatively, the image quality may be determined on the basis of subjective criteria. Here, a user determines individually as to whether or not an obtained image quality is sufficient. However, what may by all means occur in this case is that the image quality deemed sufficient by a first user is not considered sufficient by a second user. By way of example, the image quality of an image of an object may also be determined on the basis of the signal-to-noise ratio of the detector signal. The image quality is not sufficiently good in the case of a signal-to-noise ratio in the range from 0 to 5. By way of example, if the signal-to-noise ratio lies in the range from 20 to 40, this is referred to as a good signal-to-noise ratio (and hence also a good and sufficient image quality). The direction of the secondary particle beam may also be a measure for the image quality. The secondary electrons may be emitted from the object at different solid angles. Further, the backscattered electrons may be backscattered into different solid angles at the object. The direction of the secondary particle beam (i.e. the solid angles along which the secondary particle beam extend) may be influenced by tilting the primary electron beam and/or the object in relation to the optical axis of the electron beam apparatus. As a result of this, it is possible, on the one hand, to select the direction of the secondary particle beam in such a way that the secondary particle beam is incident on a desired detector. On the other hand, it is possible to influence both the number of the generated secondary electrons and the number of the back-scattered backscattered electrons by way of the aforementioned tilting. By way of example, if the primary electron beam is incident into the object parallel to a crystal lattice of an object, the number of secondary electrons and/or backscattered electrons reduces. The detection signal becomes weaker. This leads to reduction in the image quality. It is possible to increase the number of secondary electrons and number of backscattered electrons by setting the tilt of the primary electron beam. Using such a setting, it is possible to differentiate crystals with a first orientation from crystals with a second orientation on the basis of the strength of the detection signal.
As mentioned above, it is also possible to detect interaction radiation, for example cathodoluminescence and x-ray radiation. When detecting interaction radiation, a user of an electron beam apparatus may by all means be prudent to obtain the quality of the representation of the detection signals of a radiation detector based on the detected interaction radiation which is required for examining an object. By way of example, if x-ray radiation is detected by the radiation detector, the quality of the representation is determined e.g. by a good detection signal of the radiation detector. By way of example, the latter is embodied as an EDX detector. By way of example, the quality of the representation is then influenced by the count rate of the detected x-ray quanta on the one hand and, on the other hand, by the full width at half maximum of the measured peaks in the x-ray spectrum. The quality of the representation of the detection signals increases with higher count rate and smaller full width at half maximum. By way of example, if cathodoluminescence is detected by a radiation detector, the quality of the representation may likewise be determined e.g. by a good detection signal of the radiation detector. By way of example, the quality of the representation is determined by the count rate of the detected photons of the cathodoluminescence. The count rate may be influenced by a suitable optical unit for light. Further, the primary electron beam may be set in such a way that the object emits as many photons as possible overall or as many photons as possible within a specific wavelength interval.
As a rule, in order to obtain a good image quality of an image and/or a good representation of the detection signals based on the detected interaction radiation, which image and/or representation is/are generated by means of an electron beam apparatus, a user of an electron beam apparatus known from the prior art initially selects a desired landing energy with which the electrons are incident on the object. Following this, the user selects settings of further control parameters of at least one control unit. By way of example, the control parameters are physical variables, in particular a control current or a control voltage, but also e.g. the ratio of physical variables, in particular an amplification of physical variables. The values of the physical variables are adjustable at the control units or using the control units and these control and/or feed the units of the electron beam apparatus in such a way that desired physical effects, for example, the generation of specific magnetic fields and/or electrostatic fields, are brought about.
A first control parameter of a first control unit sets the contrast in the generated image. In principle, the contrast is the brightness difference (i.e. the intensity difference) between the brightest pixel with a maximum luminance Lmax and the darkest pixel with a minimum luminance Lmin in an image. A smaller brightness difference between the two pixels means a low contrast. A larger brightness difference between the two pixels means a high contrast. By way of example, the contrast may be specified as Weber contrast or as Michelson contrast. Here, the following applies for the Weber contrast:
                              K          w                =                                                            L                max                                            L                min                                      -                          1              ⁢                                                          ⁢              with              ⁢                                                          ⁢              0                                ≤                      K            W                    ≤          ∞                                    [        1        ]            
The following applies for the Michelson contrast:
                              K          M                =                                                                              L                  max                                -                                  L                  min                                                                              L                  max                                +                                  L                  min                                                      ⁢                                                  ⁢            with            ⁢                                                  ⁢            0                    ≤                      K            M                    ≤          1                                    [        2        ]            
The contrast which is substantially generated by the secondary electrons is determined by the topography of the surface of the object. On the other hand, the contrast which is substantially generated by the backscattered electrons is substantially determined by the material of the imaged object region. It is also referred to as material contrast. The material contrast depends on the mean atomic number of the imaged region of the object. By way of example, the contrast increases when a higher gain factor is set at an amplifier of the detector, wherein the detector is used to detect the secondary electrons and/or backscattered electrons. The amplifier amplifies the detection signal generated by the detector. Analogously, the contrast e.g. decreases when a smaller gain factor is set at the amplifier of the detector.
A second control parameter of a second control unit sets the brightness in the generated image. In principle, the brightness in an image is related to each pixel in the image. A first pixel with a higher brightness value than a second pixel appears brighter in the image than the second pixel. By way of example, the brightness is set by setting a gain factor of the detection signal of the detector. Here, the brightness of each pixel in the image is increased or lowered by an identical amount, for example also using a color table stored in a memory unit, with a specific brightness corresponding to a color included in the color table.
A third control parameter of a third control unit serves e.g. for actuating the objective lens, the latter being used to set the focusing of the primary electron beam onto the object.
A fourth control parameter for actuating a fourth control unit serves to center the primary electron beam in the objective lens. By way of example, the fourth control unit serves to set electrostatic and/or magnetic units of the electron beam apparatus, by means of which the centering of the primary electron beam in the objective lens is set.
Moreover, the image quality of an image of the object and/or the quality of the representation of the detection signals based on the detected interaction radiation is/are influenced by a fifth control parameter of a fifth control unit for controlling and setting electrostatic and/or magnetic deflection units which are used in the electron beam apparatus for a so-called “beam shift”. As a result of this, it is possible to set the position of the scanning region and optionally displace the scanning region to a desired position. This may occur without the use of a sample stage (also referred to as object holder below), on which the object is arranged. By way of example, if the scanning region migrates out of the actual region of the object observed by means of the electron beam apparatus on account of a change in the settings on the electron beam apparatus, the primary electron beam is displaced in such a way as a result of translational movements in the case of a “beam shift” that the scanning region once again lies in the desired observed region.
A stigmator used in an electron beam apparatus may also influence the image quality of the image of the object and/or the quality of the representation of the detection signals based on the detected interaction radiation. The stigmator—a magnetic and/or electrostatic multi-pole element—is used, in particular, for correcting an astigmatism. The stigmator may be set by a sixth control unit by means of a sixth control parameter.
The image quality of an image of the object and/or the quality of the representation of the detection signals based on the detected interaction radiation may however also be influenced by the position of a mechanically displaceable unit of the electron beam apparatus. By way of example, the image quality is influenced by the position of an aperture which is used to shape and delimit the primary electron beam in the electron beam apparatus.
The image quality of an image of the object and/or the quality of the representation of the detection signals based on the detected interaction radiation may further be influenced by the so-called scan rotation. This is a rotation of the scanning region in the plane of the scanning region about an optical axis of the electron beam apparatus.
Therefore, in order to obtain a desired image quality of an image of an object and/or a desired quality of the representation of the detection signals based on the detected interaction radiation, the user should take into account as many of the aforementioned control parameters as possible and/or further control parameters not specified here, with the physical effects obtained by the individual control parameters influencing one another in turn. The applicant is aware of the following procedures for ascertaining suitable values of the control parameters, by means of which a desired image quality and/or quality of the representation of the detection signals based on the detected interaction radiation may be obtained. By way of example, mathematical models may be used to ascertain suitable values of the individual control parameters in order to obtain a desired image quality and/or quality of the representation of the detection signals based on the detected interaction radiation. However, these calculated and theoretical values of the control parameters are often not suited to obtain a really good image quality and/or good representation of the detection signals based on the detected interaction radiation. This may be due to the fact that, for example, not all control parameters are taken into account in the mathematical models and/or the mathematical models are based on simplified assumptions which are more complicated in reality. In a further known method, provision is made for ascertaining the values of the various control parameters by experiment, with, for example, a reference sample being used for ascertainment by experiment. The ascertained values of the control parameters are used to set the control units of the electron beam apparatus. However, it is disadvantageous that an object to be examined and imaged does not always correspond to the reference sample, in particular in respect of the material composition and the topography. This may lead to optical aberrations and hence to a deterioration in the image quality which is actually obtained. A further known method lies in setting the image quality and/or the representation of the detection signals based on the detected interaction radiation by means of a manual search for the desired image quality for an object to be imaged and/or for the desired representation of the detection signals based on the detected interaction radiation. Here, the desired landing energy of the electrons, with which the electrons of the primary electron beam are incident on the object to be examined, is selected first. Subsequently, the brightness, the contrast, the focusing, the centering of the primary electron beam in the objective lens, the beam shift and/or the position of the adjustable unit are varied and matched to one another by trials in such a way until the desired image quality and/or the desired representation is/are obtained. Such a procedure is very complicated, as it has to be carried out for each setting of the landing energy.
It is therefore desirable to be able to provide a method and a particle beam apparatus for carrying out the method, by means of which values of control parameters for control units for actuating components of a particle beam apparatus are easy to ascertain, with the values of the control parameters ensuring a desired image quality of an image of an object and/or a desired representation of the detection signals based on the detected interaction radiation.